906 



and internal colour or structure of a lichen, and the kind or amount 

 of colouring matter it will be found to yield. It is exceedingly natu- 

 ral to suppose that such a ratio should exist ; but, proceeding for some 

 time on this supposition, I was frequently disappointed in my results; 

 the most showy and brilliantly- coloured lichens often furnishing the 

 dullest and most worthless colours. For instance, the bright yellow 

 thallus of Parmelia parietina, and the beautiful scarlet apothecia of 

 Scyphophorus cocciferus, instead of producing a rich yellow in the 

 one case, and a deep crimson in the other, yielded, respectively, only 

 dirty greenish-yellow and brownish colours. As a general rule, I 

 should almost be inclined to say, that the finer the colour of the thal- 

 lus of any given lichen, the more is that lichen to be suspected of 

 poverty in valuable colouring matters ; and that, on the other hand, 

 the palest pulverulent or crustaceous species, especially such as are 

 saxicolous, may be expected to yield the most beautiful and valuable 

 pigments {e. g., the Roccellas and Lecanoras). In such circum- 

 stances, it is necessary to have some test, of easy applicability, of the 

 kind and amount of colorific properties of any lichen ; and this, for- 

 tunately, is readily attainable." 



The 4th section of the paper was devoted to the consideration of 

 the various tests of colorific power which have been recommended by 

 different authors. " Of these, the greater number proceed on the 

 principle of developing the colouring matter by some alkali, in con- 

 junction with the decomposing action of atmospheric oxygen and 

 water ; others are founded on the reaction between the colorific prin- 

 ciples of certain of the dye-lichens, and some of our ordinary chemi- 

 cal reagents." The author noticed in particular — 



1. Helot's test 



2. Westring's tests h qualitative. 

 •3. Stenhouse's test 



4. „ quantitative. 



" Helof s test consists in digesting the dried and powdered lichen 

 for a few hours, at a temperature of 130°, in a weak solution of ammo- 

 nia, sufficiently strong, however, to be tolerably pungent. 



" Dr. Westring recommended simply macerating three or four 

 drachms of the liche.n in cool spring water, assisting, perhaps, the 

 solvent action of the water by minute quantities of common salt, 

 nitre, quicklime, sulphate of copper or iron, or similar reagents. If 

 these means failed after a sufficient length of time had been allowed 

 for the development of colour, he digested a fresh portion of the pul- 

 verized lichen, in water containing small quantities of sal ammoniac 



